As you breath, the air first goes to the lungs wherein the gas exchange happens. After the lungs, the blood carrying the oxygen and any other nutrients will go to the heart; first to left atrium, then to left ventricle. The blood then will leave the heart through the aorta. The blood then will be pumped to the different cells of the body. Another gas exchange happens, the oxygen from the blood will be exchange to the cell, and the carbon dioxide and other waste materials from the cell will then be exchange to the blood. The blood then now reenter the heart to the right atrium, then right ventricle and back to the lungs where carbon dioxide is breath out.